Here's a funny story..
I took melatonin 4mg daily for about a month. Stupidly I did not read up on how exactly to take it; I can be a little blase sometimes! I just took it with my vitamins in the morning at around 10am. I was at my in laws on a family visit overseas and by lunch time I would be passed out on their couch, despite having had my mornig coffee. It was kind of embarassing and I couldn't account for it, just thought I was very relaxed there. When bedtime would come I still had to take my sleeping pill (I am mentally not physically addicted I think) but I had cut it down by half. I would fall asleep very quickly and sleep right through for 8 or so hours.
Then one night as I was getting ready for bed I noticed my heart was racing. I couldn't explain it. I was feeling good, no stress or upset of any kind. It took a while to settle down but eventually it did and I went to sleep. The next night the same thing happened, but this time my heart was really racing. I woke my husband up cuz I was beginning to freak out. I googled my meds and vitamins I was taking and then thought about the melatonin.. that was the only new thing I was on. Everything else I had been on for years and "raccing pulse" didn't come up as a symptom for them. When I googled melatonin I found racing pulse was a possible side effect on one website.
My pulse did finally settle down but it was scary. I never took melatonin again. The lunch time naps ended and I didn't sleep quite as well as before.
I looked online and it seems like the consensus is that whether you take 1mg or 10mg you will get the same effect. I was also told that even .5mg should be enough. I really should try it again
in a much smaller dose AND I should try taking it an hour or so before bed, not in the morning!